Rollup merge of #81856 - Smittyvb:utf16-warn, r=matthewjasper

Suggest character encoding is incorrect when encountering random null bytes

This adds a note whenever null bytes are seen at the start of a token unexpectedly, since those tend to come from UTF-16 encoded files without a [BOM](https://en.wikipedia.org/wiki/Byte_order_mark) (if a UTF-16 BOM appears it won't be valid UTF-8, but if there is no BOM it be both valid UTF-16 and valid but garbled UTF-8). This approach was suggested in #73979 (comment).

Closes #73979.

Author: Dylan-DPC

compiler/rustc_parse/src/lexer/mod.rs

@@ -268,6 +268,9 @@ impl<'a> StringReader<'a> {
                 // tokens like `<<` from `rustc_lexer`, and then add fancier error recovery to it,
                 // as there will be less overall work to do this way.
                 let token = unicode_chars::check_for_substitution(self, start, c, &mut err);
+                if c == '\x00' {
+                    err.help("source files must contain UTF-8 encoded text, unexpected null bytes might occur when a different encoding is used");
+                }
                 err.emit();
                 token?
             }